TDDB test pattern and method for testing TDDB of MOS capacitor dielectric

ABSTRACT

A Time Dependent Dielectric Breakdown (TDDB) test pattern circuit, which can reduce testing time and statistically improve a precision of measurement as well as a method for testing the test pattern circuit are discussed. Typically, a test pattern circuit includes in plurality of unit test patterns. Each unit test pattern includes a capacitor connected to a stress voltage. The stress voltage is applied to the capacitor and the current flowing from the capacitor is measured over time. The dielectric in the capacitor breaks down over time and at a certain point, the current from the capacitor changes suddenly. Unfortunately, the convention test pattern circuit requires serial testing of each unit cell, and therefore, the measuring time is significant when there are many unit cells involved. The circuit allows for the measurements to take place simultaneously for all unit cells within the test pattern circuit. This greatly reduces the testing time, allows for greater amount of data to be obtained which improves the statistically accuracy, and reduces costs as well.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of application Ser. No. 10/895,285filed on Jul. 21, 2004, now U.S. Pat. No. 7,170,309, which in turn is aDivisional of Ser. No. 09/995,680, filed on Nov. 29, 2001, now U.S. Pat.No. 6,781,401, which in turn is a Divisional of application Ser. No.09/342,514, filed on Jun. 29, 1999, now U.S. Pat. No. 6,351,135 B1,dated Feb. 26, 2002, and for which priority is claimed under 35 U.S.C. §120; and this application claims priority of Application No. 35677/1998filed in Korea on Aug. 31, 1998 and Application No. 568/1999 filed inKorea on Jan. 12, 1999 under 35 U.S.C. § 119; the entire contents of allare hereby incorporated by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a TDDB (Time Dependent DielectricBreakdown) test pattern circuit, and more particularly, to a TDDB testpattern circuit which can reduce a test time period and improve aprecision of a measurement result statistically; and a method fortesting TDDBs of MOS capacitor dielectric films using the same.

A related art TDDB test pattern and a method for testing TDDBs of MOScapacitor dielectric films using the same will be explained. FIG. 1illustrates a unit circuit showing a related art TDDB test patterncircuit, and FIG. 2 illustrates a graph showing an experimental data ofTDDB of an MOS capacitor dielectric film measured using FIG. 1.

Referring to FIG. 1, the related art TDDB test pattern circuit includesan MOS capacitor 1 having one electrode grounded and a voltageapplication electrode 2 for applying a constant voltage V_(force) to theother electrode on the MOS capacitor 1. And, there is an ammeter 3between the MOS capacitor 1 and the voltage application electrode 2 formeasuring a current to the MOS capacitor 1.

A related art method for testing the TDDB test pattern circuit will beexplained. One side of electrode of the MOS capacitor 1 is grounded, theother side of the electrode is applied of a constant stress voltage ofVforce, and current to the MOS capacitor is measured through the ammeter3.

FIG. 2 illustrates an example of current to the MOS capacitor along timemeasured according to the above method, wherein it can be seen thatthere is a sudden increase of the current time of 340 seconds from anapplication of the voltage stress to the MOS capacitor 1. This timeperiod of 340 seconds is the Tbd(Time to breakdown) of the MOS capacitor1. In order to measure a Tbd of each MOS capacitor in N unit testpattern cells according to the test method of the related art TDDB testpattern circuit, a test for each of the N test pattern cells isrequired, i.e., N tests are required. And, if it is assumed that anaverage time period taken until an MOS capacitor dielectric film in aunit test pattern cell is Tavg, the total time required for measuring aTbd of each MOS capacitor dielectric film in N unit test pattern cellsis NxTavg.

The related art TDDB test pattern circuit and a method for testing TDDBsof MOS capacitor dielectric films using the same has the followingproblems.

Significant time period is required for Tbd testing N number of unittest pattern cells, and the use of a testing equipment for a prolongedtime period costs high.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a TDDB test patterncircuit and a method for testing TDDBs of MOS capacitor dielectric filmsusing the same that substantially obviates one or more of the problemsdue to limitations and disadvantages of the related art.

An object of the present invention is to provide a TDDB test patterncircuit and a method for testing TDDBs of MOS capacitor dielectric filmsusing the same, which can shorten a time period required for a test aswell as a time period for using a measuring equipment required for thetest and reduce a measuring cost.

Another object of the present invention is to provide a TDDB testpattern circuit and a method for testing TDDBs of MOS capacitordielectric films using the same, which allows to obtain much more testdata within the same time period, thereby improving a precision of ameasuring result in view of statistics.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, the TDDBtest pattern circuit includes a plurality of unit test pattern cellseach having an MOS capacitor, an MOS transistor, and a fuse forcontrolling operations of the MOS capacitor and the MOS transistor, afirst voltage supplying unit for supplying a stress voltage to the MOScapacitor and the MOS transistor in each unit test pattern cell on thesame time, an ammeter for continuous measurement of a total current fromthe plurality of unit test pattern cells, to measure a total time tobreakdown of the plurality of unit test pattern cells, a plurality ofVFN's(Voltage Forcing Nodes) each positioned between the first voltagesupplying unit and the fuse in the unit test pattern cell, a DCMN(DrainCurrent Measuring Node) positioned between the ammeter and a drainterminal of the MOS transistor in each of the plurality of unit testpattern cells, and a second voltage supplying unit for applying avoltage to the drain terminal of the MOS transistor.

In other aspect of the present invention, there is provided A method fortesting TDDBs of MOS capacitor dielectric films using a TDDB testpattern circuit, including the steps of (1) applying a stress voltage toan input electrode on each of an MOS capacitor and an MOS transistor ineach of a plurality of unit test pattern cells from a first voltagesupplying unit, and applying a voltage to a drain electrode of the MOStransistor from a second voltage supplying unit, (2) measuring a totaldrain current from the MOS transistor in each of the plurality of unittest pattern cells, to which the stress voltage is applied, and (3)measuring a last change of the total drain current from the MOStransistor in the plurality of unit test pattern cells, to measure amaximum time to breakdown of a plurality of MOS capacitor dielectricfilms in the plurality of unit test pattern cells.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

FIG. 1 illustrates a unit circuit showing a related art TDDB testpattern circuit;

FIG. 2 illustrates a graph showing an experimental data of TDDB of anMOS capacitor dielectric film measured using FIG. 1;

FIG. 3 illustrates a unit circuit showing a TDDB test pattern circuitusing a unit test pattern cell in accordance with a preferred embodimentof the present invention;

FIG. 4 illustrates a graph showing an experimental data of TDDB of anMOS capacitor dielectric film measured using FIG. 3;

FIG. 5 illustrates an entire circuit showing a TDDB test pattern circuitof the present invention;

FIG. 6 illustrates a graph showing an experimental data of TDDB of anMOS capacitor dielectric film measured using FIG. 5;

FIG. 7 illustrates a section showing a TDDB test pattern circuit inaccordance with a first preferred embodiment of the present invention;and,

FIG. 8 illustrates a section showing a TDDB test pattern circuit inaccordance with a second preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. FIG. 3 illustrates a unit circuit showing a TDDB test patterncircuit in accordance with a preferred embodiment of the presentinvention, FIG. 4 illustrates a graph showing an experimental data ofTDDB of an MOS capacitor dielectric film measured using FIG. 3, FIG. 5illustrates an entire circuit showing a TDDB test pattern circuit of thepresent invention, and FIG. 6 illustrates a graph showing anexperimental data of TDDB of an MOS capacitor dielectric film measuredusing FIG. 5.

Referring to FIG. 3, the unit circuit showing a TDDB test patterncircuit of the present invention includes a unit test pattern cell 10, aconstant voltage applying electrode 14 for applying a constant voltageto the unit test pattern cell 10, an ammeter 15 for measuring a currentto the unit test pattern cell 10, and a drain voltage applying electrode16 for applying a voltage to the unit test pattern cell 10. The unittest pattern cell 10 includes a MOS capacitor 11 and a MOS transistor 12operates in response to the voltage from the constant voltage applyingelectrode 14, and a fuse 13 for controlling operation of the MOScapacitor 11 and the MOS transistor 12 if an excessive current flowsfrom the constant voltage applying electrode 14. The MOS capacitor hastwo electrodes and a dielectric film formed between the two electrodes.The fuse 13 may be a metal line having a width smaller than a wiringmetal line for providing a resistance higher than the wiring metal line.The MOS transistor 12 has a gate oxide film, a gate electrode, and asource electrode and a drain electrode.

Here, the constant voltage applying electrode 14, the current meter 15,and the drain voltage applying electrode 16 are components contained ina measuring equipment for the test. There is a constant VFN (VoltageForcing Node) between the constant voltage applying electrode 14 and theunit test pattern cell 10, and a DCMN(Drain Current Measuring Node)between the MOS transistor 12 and the ammeter 15.

And, referring to FIG. 5, an overall circuit of a TDDB test patterncircuit, i.e., a main test pattern circuit includes N of unit testpattern cells connected in parallel between the constant voltageapplying electrode 14 and the ammeter 15. That is, the VFN of each unittest pattern cell is connected to the constant voltage applyingelectrode 14 and the DCMNs of unit test pattern cells are connected inparallel. And, like the TDDB test pattern of a unit test pattern cell inFIG. 3, the main test pattern includes only one of each, constantvoltage applying electrode 14, ammeter 15 and drain voltage applyingelectrode 16. In this instance, the ammeter 15 measures the sum of draincurrents from the MOS transistor 12 in each unit test pattern circuitcell.

The TDDB test pattern of the present invention having the foregoingcircuit will be explained with reference to embodiments of the presentinvention. FIG. 7 shows a first embodiment where a voltage stress isapplied to a dielectric film in an MOS capacitor 11 in an inversionmode. In this case, the type of the MOS transistor 12 is the same typeas the MOS capacitor 11. For example, as shown in FIG. 7, if the MOScapacitor 11 is an NMOS type, the MOS transistor 12 is also NMOS. And,when the voltage stress is to be applied in the inversion mode,additional source and drain is formed in the MOS capacitor 11.

FIG. 8 shows a second embodiment where the voltage stress is applied toa dielectric film in an MOS capacitor 11 in an accumulation mode. Inthis case, the type of the MOS transistor 12 should be that opposite tothat of the MOS capacitor 11. For example, as shown in FIG. 8, if theMOS capacitor 11 is an NMOS type, the MOS transistor 12 should be PMOStransistor.

Also, as shown in FIGS. 7 and 8, the main test pattern includes fourinput/output pads including a substrate pad (not shown) for connectingthe MOS capacitor 11 and a bulk electrode of the MOS transistor 12 ineach unit test pattern cell, a source pad 17 for connecting sourceelectrodes of the MOS transistors 12, a VFN pad 18 for connecting VFNsof unit test cells, and a DCMN pad 19 for connecting the DCMN connectedto the drain electrode of the MOS transistor 12 in each unit testpattern cell. In the accumulation mode, two more pads are provided to beconnected to the source and the drain of the MOS capacitor because typesof the MOS capacitor and the MOS transistor are opposite, for a total ofsix input/output pads the test pattern circuit.

Test conditions for improved testing will be explained before explaininga method for testing a TDDB of a MOS capacitor dielectric film using theaforementioned TDDB test pattern circuits of the present invention. Afirst condition for improved testing using the TDDB test patterncircuits is to form the gate dielectric film of the MOS transistor 12thicker than a thickness of a dielectric film of the MOS capacitor 11.The second condition to dope the gate electrode of the MOS transistor 12lighter than the MOS capacitor 11. If the gate oxide film of the MOStransistor 12 is thicker than the dielectric film of the MOS capacitor11, the gate oxide film of the MOS transistor 12 operates normallywithout being influenced from the voltage stress during a time periodbefore the dielectric film in the MOS capacitor 11 breaks due to thevoltage stress, because the Tbd is sensitive to the thickness of thedielectric film; a small increase of the thickness of the dielectricfilm increases the Tbd significantly under the same voltage stresscondition.

Also, the light doping of the gate electrode of the MOS transistor 12causes a voltage significantly lower than the V_(force) to be applied tothe gate oxide film due to depletion of the gate electrode. In thisinstance, because the voltage stress is low, the Tbd of the MOStransistor 12 is increased significantly even if the thicknesses of thedielectric film of the MOS capacitor 11 and the gate oxide film of theMOS transistor are the same. Therefore, the MOS transistor 12 operatesnormally because the gate oxide film of the MOS transistor 12 has verylittle stress exerted thereon until a breakdown of the dielectric filmin the MOS capacitor 11 occurs due to the voltage stress.

Next, a method for testing a TDDB of a dielectric film in the MOScapacitor 11 using the TDDB test pattern circuit will be explained.

Referring to FIG. 8, a constant voltage V_(force) is applied to the MOScapacitor 11 and the MOS transistor 12 in the unit test pattern cell 10from the constant voltage applying electrode 14. In this instance, thereis a leakage current flowing through the dielectric film of the MOScapacitor 11 and the gate oxide film of the MOS transistor 12. However,the ohmic voltage drop of the fuse 13 negligible, because the leakagecurrent is very low. Thereafter, as explained before, the voltage stresscauses a breakdown only on the dielectric film of the MOS capacitor forthe testing time period causing a large current flowing through the fuse13.(see FIG. 2). This current generates a Joule's heat against aresistance of the fuse 13, which breaks the fuse 13. Consequently, nomore voltage stress V_(force) is applied to the gate electrode of theMOS transistor 12. The dielectric film in the MOS capacitor 11 loses itscapacitive characteristic after the breakdown, and a ground voltage isapplied to the gate electrode of the MOS transistor 12 through the MOScapacitor 11. Accordingly, the MOS transistor 12 turns off and thecurrent flowing to the drain electrode drops suddenly due to a voltagechange(V_(force) ³0) at the gate. That is, the breakdown of thedielectric film of the MOS capacitor 11 is directly measured as thedrain current drop of the MOS transistor 12. In this instance, thevoltage applied to the drain of the MOS transistor 12 may be in either alinear region or a saturation region, a low voltage is favored whentaking a possibility of performance degradation of the MOS transistor 12caused by a hot carrier effect and a reliability into account.

As shown in FIG. 4, Tbd characteristics of such a unit test pattern cellexhibits a sudden drop of a drain current Id after a time period ‘t ’when V_(force)>0 is applied as the stress to the unit test pattern cellof the first embodiment, and when V_(force)<0 is applied as the stressto the unit test pattern cell of the second embodiment. The time period‘t’, being a time period required for a change of the drain current(Id),is a Tbd(Time to breakdown) of the dielectric film in the MOS capacitor11.

Next, a method for testing a TDDB using the main test pattern circuithaving N unit test pattern cells connected in parallel will beexplained.

As shown in FIG. 5, constant voltage V_(force) is applied to each of theN unit test pattern cells through respective VFN from the constantvoltage applying electrode 14. In this instance, the constant voltageV_(force) is applied to the MOS capacitor 11 and the MOS transistor 12in each unit test pattern cell simultaneously. Thereafter, each unittest pattern cell operates as the unit test pattern cell explained aboveregardless of operations of other cells. A TDDB test operation of themain test pattern having N unit test pattern circuit cells proceeds bymeasuring times at each of which a sum of the drain currents from the NMOS transistors are reduced following a breakdown an the dielectric filmof each MOS capacitor. That is, upon breakdown of arbitrary MOScapacitor of the N unit test pattern cells, the sum of the draincurrents from entire MOS transistors is reduced.

FIG. 6 illustrates a result expected change of the sum(Id(total)) of thedrain currents along a time axis. As shown at times t1, t2, t3, - - - ,tN at which the sum of the drain currents is reduced represents a Tbd ofan arbitrary dielectric film in the MOS capacitor. The tN, representinga maximum time period tmax required for testing all the N unit testpattern cells, is the Tbd of the unit test pattern cell which takes thelongest breakdown.

The TDDB test pattern and the method for testing TDDBs of MOS capacitordielectric films using the same have the following advantages.

First, the time period required for testing all the N unit test patterncells is a Tbd of the unit test pattern cell which takes the longest tobreakdown. Therefore, in a case where numerous cells are involved, thetime required for measuring the same can be reduced substantially.

Second, the reduction of testing time reduces measuring cost.

Third, from a statistical perspective, a substantially greater data canbe obtained, which improves a precision of the measurement result inview of statistics.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the TDDB test patterncircuit and the method for testing TDDBs of MOS capacitor dielectricfilms using the same of the present invention without departing from thespirit or scope of the invention. Thus, it is intended that the presentinvention cover the modifications and variations of this inventionprovided they come within the scope of the appended claims and theirequivalents.

1. A method for testing Time Dependent Dielectric Breakdowns (TDDBs) ofmetal-oxide-semiconductor (MOS) capacitor dielectric films using a TDDBtest pattern, the method comprising: providing a plurality of unit testpattern cells between a first node and a second node, each of the unittest pattern cells including a fuse coupled between the first node and athird node, a MOS capacitor coupled between the third node and a ground,and a MOS transistor coupled between the ground and the second node,wherein a gate of the MOS transistor is coupled to the third node;applying a first voltage to the first node and applying a second voltageto the second node and providing an ammeter between the second node anda second voltage supplying unit that supplies the second voltage; andmeasuring a total drain current using the ammeter.
 2. A method asclaimed in claim 1, wherein the MOS transistor is an N type when the MOScapacitor is the N type.
 3. A method as claimed in claim 2, wherein thefirst voltage is higher than zero (0) volt when the MOS transistor andthe MOS capacitor are the N type.
 4. A method as claimed in claim 1,wherein the MOS transistor is an N type when the MOS capacitor is a Ptype.
 5. A method as claimed in claim 4, wherein the first voltage islower than zero (0) volt when the MOS transistor is the N type while theMOS capacitor is the P type.